In modern Wavelength Division Multiplexed (WDM) optical communications networks, it is a common practice to utilize passive MUX/DEMUX devices (for example using thin film filters) or dynamic switching (for example using Wavelength-selective switches and channel blockers) to control the routing of wavelength channels through the network.
A limitation of passive devices is that they are relatively static, in that the routing of wavelength channels is controlled by the physical structure of the filter. This means that in order to implement a change in the routing it is necessary to change the filter. In the case of an wavelength channel carrying customer data, there is no simple method of changing the routing without disrupting the customer data flow, which is highly undesirable.
Dynamic wavelength switching allows for a readily reconfigurable wavelength routing across the network. However, Wavelength-selective switches (WSSs) are expensive and require control software to correctly manage the state of the switch.
Alternative network architectures are known that do not perform any wavelength routing or blocking functions. In such architectures, wavelength channels are not routed across the network. Rather, they are effectively broadcast. FIG. 1 schematically illustrates a representative network architecture of this type. In the network of FIG. 1, a set of four nodes 2A-D are shown, each of which is connected to its neighbor by a pair of unidirectional fiber links 4L and 4R. Within each fiber link 4, WDM optical signals are propagated in the direction illustrated by arrows. Each node 2 comprises a Drop path 6 for coupling inbound light from a respective upstream fiber link 4 to a receiver 8, and an Add path 10 for coupling light from a transmitter 12 to a respective downstream fiber link 4. Typically, a transmitter 12 and a receiver 8 are combined into a transceiver 14, which is configured to transmit and receive optical signals at the same wavelength. Normally, each node 2 will comprise multiple transceivers 14, each of which configured to operate (that is, transmit and receive) at a respective different wavelength. For ease of illustration, only one transceiver 14 shown in each node 2.
The Drop path 6 comprises an optical coupler 16 (such as a 3dB splitter) for tapping light from a respective upstream fiber path and supplying the tapped light to the receiver of a transceiver. Typically, a node 2 will include multiple transceivers 14, in which case, the Drop path 6 may also include one or more optical splitters (not shown) for supplying Drop light to the respective receiver of each transceiver. As may be appreciated, the Drop path 6 supplies light of all of the channels active in the upstream fiber path 4 to the (or each) receiver 8. Accordingly, the receiver must be tuned to receive a selected one of the wavelength channels. In the case of coherent receivers, tuning can be achieved by suitable control of local oscillator in a manner known in the art. In the case of direct detection receivers, filters may be used to limit the spectral range of light admitted to the receiver 8.
The Add path 10 comprises an optical combiner 18 for adding light from the transmitter 12 of a transceiver 14 to a respective down-stream fiber path. Typically, a node 2 will include multiple transceivers 14, in which case the Add path 10 may include one or more optical combiners (not shown) for combining light from the respective transmitter of each transceiver. Preferably, the transmitter 12 of each transceiver 14 can be tuned to transmit within an optical channel centered on a desired wavelength. Various means of tuning transmitters in this manner are known in the art.
With the arrangement of FIG. 1, signals transmitted from any given node will propagate through the corresponding downstream fiber links and will be dropped to each receiver(s) coupled to the downstream fiber link. In the example, of FIG. 1, optical signals transmitted from Node A will propagate through the fiber link 4R, and be dropped to the receivers 8 in nodes C and D. Similarly, optical signals transmitted from Node C will propagate through the fiber link 4L, and be dropped to the receivers 8 in nodes A and B. This means that a communication link between node B and either one of nodes C and D can be established by tuning a respective transceiver in each node to a common wavelength. For example, FIG. 1 illustrates a scenario in which Nodes B and D are configured to communicate with each other using a wavelength channel identified as λ1. In the illustrated scenario the transmitter at Node B transmits an optical signal on λ1, which is added to Node B's downstream fiber link. As may be seen in FIG. 1, λ1 propagates through the fiber link 4R, and is coupled into the respective Drop path 6 of each of nodes C and D. As a result, bi-directional communication between Nodes B and D can be established by configuring a transceiver 14 in each node to operate at λ1.
An advantage of this architecture is that it eliminates the expense and complexity of conventional wavelength routing equipment. However, it also creates failure mechanisms that do not exist in conventional network architectures. A specific problem is that an optical signal transmitted by one node is effectively broadcast to every node in the network, not just the intended recipient node. This is illustrated in FIG. 1, in which λ1 is used to establish communication between nodes B and D, but is also propagated to nodes A and C as well. In the absence of wavelength channel routing or blocking devices, there is no means of preventing propagation of a wavelength channel to every node in the network. However, if the network has no capability for blocking wavelengths, then there is no means of preventing wavelength collisions in the network. For example, if a transceiver 14A in node A were to begin transmitting at λ1, then those signals would “collide” with the signals being exchanged between nodes B and D, and so interfere with communications between these two nodes.
In addition to the aforementioned optical network architecture with no wavelength blocking, the potential for wavelength conflict exists in conventional wavelength-routed networks which use a wavelength-independent power combiner as part of the multiplexing scheme. For example, in an Optical Add/Drop Multiplexer (OADM) it may be beneficial to use a wavelength-independent power combiner (e.g. having N input ports) to multiplex N transmitter outputs to a single fiber. However, with this arrangement, a wavelength collision can occur if a newly activated transmitter starts emitting light that spectrally overlaps with light emitted by a previously activated transmitter connected to the same N-port combiner. Such a collision would cause the previously activated transmitter's signal to be degraded by the interfering signal of the newly added transmitter.
As is known in the art, a “wavelength collision” refers to the insertion of a signal onto a network at a wavelength that is already in use by a pre-existing connection. The newly introduced signal would interfere with the pre-existing signals and would disrupt the pre-existing connection. It is therefore advantageous for any optical network to have the capability for avoiding wavelength collisions.
Typically, this problem is addressed by enforcing a network management policy of using a particular wavelength only once in the network. However, there is still a risk of disruption of an operational communications link, if a new link is erroneously activated at the same wavelength as the pre-existing link.
Conventional approaches to wavelength collision avoidance in networks without wavelength routing use a combination of channel blockers (for example at mux ports) and a spectral validation scheme. For example, when a wavelength channel is to be added to the network, the selected wavelength is first validated through either a direct measurement (e.g. using an Optical Signal Analyser) or through provisioning. The wavelength channel is then compared against a database, frequently stored within the local nodes' central processing unit, to determine if the wavelength is available for insertion on the network. If the database entry for that wavelength indicates that the wavelength is available, then the channel blocker at the mux port is released and the database updated to indicate that the newly inserted wavelength is now in use. However, this approach relies on multiplexers with port-level channel blockers, which tend to be expensive. In addition, the addition of the new wavelength channel must be communicated other nodes, either by OSA signalling or manually. Automated notification via OSA signalling increases signaling between nodes and requires that the Photonic Line have management visibility to transmitters, which may be part of a different product and/or managed independently. On the other hand, manual updates of local node databases are prone to human error.
Techniques that overcome at least some of the above-noted limitations of the prior art would be highly desirable.